You executed Split Window Horizontally from menu Window.
Splitting a document window horizontally to be able to view/edit on two different positions in same file can be also done with mouse by clicking with primary (left) mouse button on small rectangle box above the vertical scroll bar, hold the mouse button, drag down the mouse with the dotted line and leave the mouse button, see attached image SplitWindowHorizontallyBegin.png.
To end the split window mode click with primary (left) mouse button on window border area above the ruler, hold the mouse button, drag up the mouse with the dotted line to top of the split document window and leave the mouse button, see attached image SplitWindowHorizontallyEnd.png.
It is also possible to end the document window split by saving and closing the file and re-open it.
This feature is a common feature which many Windows application support. The same feature have also Microsoft Word or Adobe Reader and is quite often very useful, especially on source code writing which is the reason why I'm explaining it in detail here.